Increased recovery of the metal values of junked cars began when the steel mills and iron foundries required and demanded a higher quality steel scrap. This was accomplished by shredding the car in a flying hammermill to liberate the mechanically bound constituents of the car from each other. Once liberation is achieved, the magnetic ferrous material is separated by the use of a magnetic field of low intensity, producing a valuable high quality iron scrap and a poor quality mixture of non-magnetic metals, plastics, and rubber. Aluminum, zinc, copper, brass, stainless steel and lead are valuable constituents of this non-magnetic shredded automotive scrap and several techniques have been developed for their recovery using the significantly different material densities by heavy media separation. This beneficiation technique takes advantage of low specific gravity, 2.7 grams per cubic centimeter of aluminum by introducing the above described scrap metal mix into a slurry consisting of water with suspended particles of ferrosilicon. When there is a sufficient quantity of ferrosilicon in the slurry, it will appear to the scrap to be a homogeneous fluid with a specific gravity greater than that of aluminum. As a result, the aluminum floats on top of the heavy media (i.e., water and ferrosilicon slurry) while the other non-magnetic constituents, i.e., zinc, copper, brass, lead, and stainless steel sink, thus facilitating the aluminum's separation from the scrap mix.
The metals which sink in this process are heavy non-magnetic materials and after removal from the heavy media ferrosilicon vessel, they are normally passed over a screen with openings of about three-quarters of an inch. The pieces of oversize (i.e., larger than 3/4 inches) are then deposited on a conveyor belt where the zinc, copper, brass and lead are separated from each other by hand. The pieces of undersize (i.e., smaller than 3/4 inches) are too small for hand sorters to effectively identify and hand sorting of this small material becomes uneconomical.
Several pyrometallurgical and hydrometallurgical techniques are available for recovery of the metal values in these undersize mixtures of small size fraction materials, more particularly the recovery of metal from this material mixture, is aimed at obtaining zinc from the mixture since it represents the largest component remaining.
Zinc possesses two properties that would appear to lend it to techniques for its separation from the non-ferrous scrap mix but unfortunately significant shortcomings in these techniques are apparent.
A pyrometallurgical technique which can be used with some success is selective melting. The zinc and zinc alloys in the mixture which have aluminum and copper as the alloying constituents, have melting temperatures on the order of 800.degree. F. which is much lower than all the other non-magnetic constituents, which have melting temperatures which are greater than 1200.degree. F., except for the lead and lead alloys present, which melt at about 620.degree. F. It would appear that first the lead and then the zinc could be preferentially melted from the scrap mix and thus result in an effective metal separation. However, due to the small amount of lead present, about 4% by weight of the total mixture, and the manner in which the pieces of scrap pack together when introduced to the sweating furnace, the molten lead will not readily flow out of the scrap mixture. The result is that when the zinc is melted the residual lead will alloy with the molten zinc and contaminate it. Since the lead specification for zinc and zinc alloys is 0.005% maximum (see specification ASTM B86 for alloys AG40A and AC41), the resulting zinc product is not satisfactory from this selective melting process. Furthermore, significant metal losses due to oxidation are usually associated with preferential melting.
A hydrometallurgical technique thought to be of advantage for the separation of zinc from the metal mixture utilizes an acid environment where the zinc will act as a sacrificial anode and dissolve preferentially to the other more noble metals in the scrap mixture the dissolved zinc can then be recovered from the aqueous solution by electrowinning. Though technically feasible this hydrometallurgical technique would involve the handling of large quantities of acid, probably sulfuric, and require several unit operations such as dissolution, liquid-solid separation, solution purification, and electrowinning. Also to be considered is that even though the product of such a process would be a very pure metal, about 99.99% zinc, it is no more valuable than the alloys which were dissolved.
From the above discussion it is apparent that more suitable techniques are required to separate the valuable zinc and zinc alloy materials from the mixture of small size materials.